admin.php tries to read $plugin_page from the wrong directory

Description

2.9 did this right. It should do

include(WP_PLUGIN_DIR . "/$plugin_page");

instead of

include(ABSPATH . PLUGINDIR . "/$plugin_page");

on line 155 of wp-admin/admin.php

It looks like a bug that got introduced by merging WPMU code.
This breaks the features introduced in 2.6 that let you keep WordPress in its own directory separate from wp-content (if WP_CONTENT_DIR != ABS_PATH . '/wp-content')